Throughout this application various publications, patents, patent application publications and books are referred to. Full citations for these may be found at the end of the specification. The disclosures of these publications, patents, patent application publications and books are hereby incorporated by reference in their entirety into the subject application to more fully describe the art to which the subject invention pertains.
From laboratory to clinic, directing therapeutics to specific sites (cells, tumors, tissues) and ensuring that the delivered agents will be active only at the desired sites remains a daunting challenge. Targeting is a particularly critical issue in cancer therapy because of the undesirable side effects of chemotherapeutic agents, many of which can also be toxic to normal cells.
The transferrin receptor, CD71 (TfR), is one of the mostly widely targeted receptors for development of targeted cancer diagnostics and therapeutics [1]. This type II transmembrane glycoprotein is responsible for cellular iron transport and is found at low levels on the surface of many normal cell types. However, the receptor is highly expressed on cells with increased proliferation rates. In particular, increased expression is observed across a wide range of cancer cells where increased expression is associated with poor prognosis [2]. Perhaps most interestingly, the receptor is also expressed at high levels on the blood brain barrier where it has been shown to be a route for ferrying cargoes across the blood brain barrier [3].
The natural ligand of this receptor, transferrin, has been used extensively for the generation of numerous targeted drug delivery strategies including the development of targeted toxins, some of which have gotten as far as clinical trials [4-7]. Aside from anti-integrin binding RGD peptides, transferrin is perhaps the most commonly used targeting ligand for functionalizing nanoparticles for nanoparticle-based diagnostics and therapeutics [8, 9]. Nanoparticles in and of themselves have a tendency to accumulate in the fenestrations of tumors through a phenomenon known as the enhanced permeability and retention (EPR) effect [10]; however, greater gains, in particular extravasation and tissue penetration, can be achieved by including targeting ligands on the surface of nanoparticles. Specifically, transferrin has previously been shown to enhance the effect of nanoparticle based siRNA delivery in vivo [11].
In addition to the natural ligand, transferrin, a variety of antibodies [1] and even peptides [12] have been identified which target the transferrin receptor. More recently, Chen et al. reported the development of both RNA as well as DNA aptamers which target the murine transferrin receptor and demonstrated that a DNA aptamer-alpha-l-iduronidase conjugate taken up by alpha-L-iduronidase-deficient mouse fibroblasts cells were transported to lysosomes where they alleviated glycosaminoglycan accumulation [13]. The selected aptamers were specific for the mouse receptor but were, unfortunately, composed of natural nucleotides rendering them highly susceptible to degradation by serum nucleases and thus limiting their utility.
The present invention provides aptamers are engineered to target specific cells and methods of targeted delivery using aptamers.